Bacteriophages (or phages) are small viruses displaying the ability to infect and kill bacteria while they do not affect cells from other organisms. Initially described almost a century ago by William Twort, and independently discovered shortly thereafter by Félix d'Herelle, more than 6000 different bacteriophages have been discovered so far and described morphologically, including bacterial and archaeal viruses. The vast majority of these viruses are tailed while a small proportion are polyhedral, filamentous or pleomorphic. They may be classified according to their morphology, their genetic content (DNA vs. RNA), their specific host, the place where they live (marine virus vs. other habitats), and their life cycle. As intra-cellular parasites of bacterial cells, phages display different life cycles within the bacterial host: lytic, lysogenic, pseudo-lysogenic, and chronic infection (Weinbauer, 2004; Drulis-Kawa, 2012). Lytic phages cause lysis of the host bacterial cell as a normal part of their life cycle. Lysogenic phages (also termed temperate phages) can either replicate by means of the lytic life cycle and cause lysis of the host bacterium, or they can incorporate their DNA into the host bacterial DNA and become noninfectious prophages. Whatever the type of cycle of a phage life, the first step is the attachment to receptors of the bacterial cell wall before phage material may enter the bacteria. This specific process influences the spectrum of the possible phage-bacteria interactions.
Bacteriophages are commonly used as research tools to modify bacteria in laboratory experiments.
Because of their target host cell specificity, the use of phages as a therapy to treat acute and chronic infections has been considered, particularly in dermatology, ophthalmology, urology, stomatology, pediatrics, otolaryngology or surgery. This concept of therapeutic use of phages to treat bacterial infection was, however, highly controversial from the very beginning and not widely accepted by the public or medical community. Early studies were widely criticized for lack of appropriate controls and inconsistent results. The lack of reproducibility and many conflicting results obtained in the various published studies led the Council on Pharmacy and Chemistry of the American Medical Association to conclude that the evidence for the therapeutic value of lytic filtrates was for the most part contradictory and unconvincing, and recommend additional research to confirm its purported benefits.
Since the introduction of antibiotics in the 1940s, little attention was paid to this field of therapeutics, especially in the Western world. But the extensive use of antibiotics has led to the widespread emergence and spread of antibiotic-resistant bacteria around the world, causing increasingly serious problems. It has therefore become a major therapeutic challenge to overcome the limited therapeutic options remaining to treat major multi-drug resistant microbes.
In addition, many pathogenic microorganisms reside within biofilms, which biofilms cause additional problems when designing new anti-microbial agents. In this regard, bacteria growing as a biofilm rather than in single-celled (“planktonic”) forms tend to be particularly resistant to anti-microbial agents and to be particularly difficult for the host immune system to render an appropriate response.
Since its initial discovery in the late 19th century (Fordos 1859), the Gram-negative bacterium Pseudomonas aeruginosa has gained a notorious place in the list of infamous human pathogens (Williams et al., 1894; Freeman et al., 1916). The arrival of the antibiotic era largely palliated the previously fatal outcome of acute infections in healthy patients. Only a relative improvement has been achieved in the eradication of chronic infections, which develop mainly in individuals suffering from cystic fibrosis or severe burns or who are immunocompromised (Gang et al., 1999; Jones et al., 2010). Two intrinsically related factors in the fatal outcome of infection in these patients are the rapid prescription of inappropriate antibiotic treatments and the development or acquisition of multidrug-resistant strains. While the use of (an) appropriate antibiotic(s) has been reported as an essential factor in the eradication of P. aeruginosa infections (Kang et al., 2005; Micek et al., 2005), conversely, antibiotic abuse significantly contributes to increasing resistance by exerting a continuous selective pressure for the acquisition of such capabilities. Antibiotics alone do not account for the high prevalence of multidrug-resistant variants: P. aeruginosa has multiple, chromosomally encoded intrinsic mechanisms of resistance, including low permeability of the cell envelope and numerous multidrug efflux pumps. Another major factor accounting for the successful invasive behavior and persistence of this bacterium is its high adaptability, allowing rapid colonization of different environments.
Furthermore, pathogenic bacteria such as P. aeruginosa are able to form biofilms, which contribute to their increased resistance to antibiotics. Such biofilms may comprise more than one type of bacteria supported and surrounded by an excreted extracellular matrix, and assist bacteria to colonize various surfaces. Biofilms allow bacteria to attach to surfaces and to reach population densities which would otherwise be unsupportable, imparting increased resistance to not only antibiotics but many environmental stresses including toxins such as heavy metals, bleaches and other cleaning agents. It is known that bacteria within biofilms can be 100 to 1000 times more resistant to antibiotics than the same strain of bacteria growing in planktonic forms. Such an increased resistance means that bacteria that are apparently sensitive to antibiotics in a laboratory test may be resistant to therapy in a clinical setting. Even if some are cleared, biofilms may provide resistant reservoirs permitting rapid colonization once antibiotics are no longer present. It is therefore obvious that biofilms are major factors in many human diseases. Chemical treatments are unsuited for use against biofilms since this is precisely what they have evolved to counter. Physical abrasion does provide a means to disrupt biofilms. Unfortunately, many surfaces where biofilms support bacterial pathogenesis are poorly suited to rigorous abrasion, i.e., bones, joints, implanted medical devices, etc. For example, the surfaces of wounds or burns are extremely sensitive and delicate. Even where abrasion is both suitable and in routine use, clearing of biofilms is limited. Oral plaque on the surface of teeth is a biofilm and is partially cleared by regular brushing. However, bacteria are maintained on unbrushed surfaces (for example in the gaps between teeth) and can recolonize cleared surfaces both rapidly and effectively. From this, it is clear that existing approaches to clearing biofilms are of limited efficacy.
The capability for quick adaptation and the ability to form biofilms are the main reasons that identify P. aeruginosa as opportunistic pathogens. They have acquired the status of hospital pathogens, and may be isolated from clinical samples taken from wounds, sputum, bladder, urethra, vagina, ears, eyes and respiratory tract. The emergence of resistance to the most powerful new antibiotics in such clinical P. aeruginosa strains, occurring even during treatment, makes the fight with P. aeruginosa hospital pathogens a great problem.
Furthermore, it has been reported that the pathological or physiological condition of a subject is influenced by the balance of microorganisms in the flora of the subject. Accordingly, modifying the microbial flora, or modifying said balance, or restoring said balance, by destroying P. aeruginosa population, is also a valuable approach for improving a subject condition.
Therefore, there is a great need for new antibacterial agents or compositions that can be used to destroy P. aeruginosa strains, even when organized in bacterial biofilms, suitable for use in human or animal therapy, as well, as for decontaminating materials.